[BeckyEbook]

My experiments don't always lend themselves to wider use, and then I'm the only person in the world who uses such non-standard settings, because, after all, it's mostly about individual taste.
I work in Sigil practically every day, so my preferences are simply due to the mode of this work, which is why, among other things, I have a validation results box above Code View, because I mostly analyze the results of validation plugins.
It is also important to note that my changes are more about Windows in a Classic Sigil Dark Theme.

/* Setting the monotype font in the Find and Replace fields */
/* Instead of "Consolas" use your favorite font, preferably the one you use in Code View. */
#cbFind, #cbReplace {font-family: "Consolas"; font-size: 12px;}

/* Change the default selection color to a similar green color scheme for the new logo/icon. */ 
*,
*::separator,
*::handle,
*::tab-bar,
*::tab,
*::section {
  selection-background-color: #00404a;
  selection-color: white;
}

/* Sigil version number in About box in different color */
QLabel#lbVersionDisplay {color: #FFA000;}

/* Gray background in About box on Authors tab */
QTextEdit#creditsDisplay {background-color: #323232;}

/* Gray background of the BookBrowser panel */
#bookbrowser {background-color: #323232;}

/* No BookBrowser panel borders  */
#bookbrowser {border: 1px solid #323232;}

/* No Find&Replace panel borders */
QFrame {border: 1px solid #323232;}